The issue with waiting for 6 years is that his siblings have been taking care of him for that long. You are sponsoring him as an orphan. You wait to be approved to sponsor.
These applications are quite rare and applications from Pakistan tends to have longer processing times so would expect at least a year probably. When will he be 17? If his siblings have been his guardian for the past 6 years I assume they must be quite a bit older than him and was there not a plan for his care when they decided to move? Given the timeline and age there may be concerns that this sponsorship is for immigration purposes not for caregiving reasons.
